Ink jet printer with secondary, cyclically varying deflection field

ABSTRACT

An ink jet printer transporting a print receiving medium angularly past a print head which generates a row of jet drop streams. Drops in the streams are selectively charged to either a print charge level or a catch charge level, and are subsequentlysubjected to a cyclically varying electric deflection field, normal to the row, which deflects of the drops carrying the print charge level into selected print trajectories. A static electric deflection field then separates the drops into print and catch trajectories, with the drops in the catch trajectories being intercepted by a catcher and thereby prevented from striking the print receiving medium. Drops from each jet drop stream may be deflected in a direction perpendicular to the row by the secondary field so as to be deposited at a number of positions on the print receiving medium.

BACKGROUND OF THE INVENTION

The present invention relates to jet printing devices and, moreparticularly, to apparatus for controlling accurately the deposit of inkdrops on a print receiving medium to produce collectively an image ofhigh resolution.

Jet drop recorders, such as that shown in U.S. Pat. No. 3,701,998,issued Oct. 31, 1972, to Mathis, have included one or more rows oforifices which receive electrically conductive recording fluid, such aswater base ink, from a pressurized fluid manifold and eject the fluid asparallel fluid filaments. Mechanical stimulation is applied to thestructure or is fluid-coupled to the orifices, causing each of thefilaments to disintegrate into a jet drop stream.

Graphic reproduction in recorders of this type is accomplished byselectively charging some of the drops in each of the streams. The dropsthen pass through an electric field which deflects the charged dropssuch that they strike a drop catching device. The uncharged drops,however, pass unaffected through the deflection field and are depositedon a moving web of paper or other material. The uncharged dropscollectively form a print image on the web.

A problem with jet printers has been attaining sufficient imageresolution. Since a discrete number of drops from the printed images, itis clear that an increase in the number of drops deposited per unit areaof print medium, and a corresponding increase in data handlingcapability, permit improved image definition. If, however, each jet isused in a binary manner to deposit drops selectively at a singleassociated position on the print receiving medium, the number of dropsper unit width and, therefore, the resolution of an image in thedirection transverse to the print web, are limited by the minimumdimensions required between orifices. U.S. Pat. No. 4,010,477, issuedMar. 1, 1977, to Frey discloses a printer in which the effective densityacross the print receiving medium is increased by orienting rows of jetsalong angularly positioned placement lines. The Mathis '998 deviceincreased the number of drops across the width of the medium by usingmultiple rows of jet drop streams which interlace. The printer disclosedin "Ink Jet Head," by Krause, IBM Technical Disclosure Bulletin, Vol.19, No. 8, January 1977, pp. 3216 and 3217, combines these approaches byproviding two interlaced rows of drop streams which are positionedobliquely with respect to the direction of web movement.

It will be appreciated, however, that increased resolution may beobtained by constructing the printer such that drops from each jet canbe deflected selectively to any of a number of positions on the printreceiving medium. U.S. Pat. No. 3,739,395, issued Jun. 12, 1973, to Kingdiscloses a printer in which uncharged drops are caught while thecharged drops from each orifice are deflected by two sets of deflectionelectrodes to a plurality of discrete positions on the moving web.Deflection of the drops is either perpendicular or parallel to thedirection of web movement, or both, covering either a one line matrix ora multiple line matrix across the web. The minimum distance between jetorifices is somewhat greater in the King device than in previouslymentioned devices, however, since deflection electrodes must bepositioned on all sides of each orifice.

U.S. Pat. No. 3,972,052, issued Jul. 27, 1976, to Atumi et al disclosesan ink jet printing device in which a single jet scans a plurality ofprint lines in succession under control of two pairs of deflectionelectrodes. The electrodes provide parallel deflection fields throughwhich the ink drops pass in succession, with identical ramp deflectionvoltages being applied to the deflection electrodes. The deflectionvoltage applied to the second pair of deflection electrodes is delayedwith respect to the deflection voltage applied to the first pair so thatthe same linearly increasing field is experienced by each drop betweenboth pairs of electrodes.

The difficulties encountered with structures in which deflectionelectrodes are positioned intermediate adjacent jet drop streams havebeen eliminated by providing a print head which generates a plurality ofjet drop streams positioned along a row which is inclined with respectto the direction of web movement, and in which deflection of the dropsin the streams to various print positions is accomplished by adeflection field or fields perpendicular to the row or rows of streams.Such arrangements are shown in U.S. Pat. No. 4,085,409, issued Apr. 18,1978, to Paranjpe and U.S. Pat. No. 4,122,458, issued Oct. 24, 1978, toParanjpe. In the Paranjpe '409 device, drops are selectively charged todiffering print charge levels such that the degree of deflection whichthey experience as they pass through the deflection field variescorrespondingly. In the Paranjpe '458 patent, a printer is disclosed inwhich the trajectories of the drops are controlled by varying,non-cyclical electric deflection fields. The fields are altered independence upon the amount of deflection dictated by the printcontrolled data.

It can be seen, therefore, that in the Paranjpe '409 and Paranjpe '458patents, the printers require either charging of drops to multiplediscrete levels or non-cyclical high speed of the fluctuation deflectionfield. Large swings in field intensity are not easily accomplished inshort periods of time due to the capacitive nature of the deflectionelectrodes. Additionally, it will be appreciated that providing aplurality of discrete deflection electrodes for each jet drop stream mayresult in substantial cross talk between the deflection fieldcontrolling adjacent jets. High speed fluctuation in charge potentiallevels, on the other hand, where many such levels are used, as in theParanjpe '409 device, is complex and requires substantial controlcircuitry.

It is seen, therefore, that there is a need for a simple printer capableof high speed printing with a large number of ink drops so as toincrease image definition.

SUMMARY OF THE INVENTION

An ink jet printer according to the present invention includes means fortransporting a print receiving medium past a print station, a print headmeans for generating a row of jet drop streams directed at the medium atthe station, the row being inclined with respect to the direction ofmovement of the medium, and charging means for selectively chargingdrops in the streams such that selected ones of the drops carry a printcharge level and others of the drops carry a catch charge level. Theprinter further includes means for generating a cyclically varyingelectric deflection field, normal to the row of jet drop streams, fordeflecting drops carrying the print charge level into selected printtrajectories, a means for generating a static electric deflecting fieldfor separating the drops into print and catch trajectories, and catchermeans, positioned to intercept drops in the catch trajectories andprevent the intercepted drops from striking the print receiving medium.

The charging means may include means for inducing a non-zero printcharge level. The charging means may further include means for inducinga non-zero catch charge level which differs substantially from thenon-zero print charge level. Alternatively, the charging means mayinclude means for providing a zero catch charge level.

The means for generating a cyclically varying electrical deflectionfield may include means for subjecting only one print drop in eachstream to the cyclically varying electrical deflection field at anygiven time. The means for generating a cyclically varying electricaldeflection field may further include shielding means for limiting thefield.

The ink jet printer utilizing a non-zero catch charge level may have thecatcher positioned to one side of the row of jet drop streams andextending parallel to the row. The means for generating a cyclicallyvarying field may include deflection electrodes and means for applying acyclically varying electrical potential to the deflection electrodes.The field may be varied in a stepwise manner.

Accordingly, it is an object of the present invention to provide an inkjet printer in which drops are given a first, non-zero charge level whenthey are to be deposited on the print receiving medium and in which theyare given a second charge level when they are to be caught and in whichdrops from each jet drop stream may be deposited at a plurality of printpositions on the medium; to provide such an ink jet printer in which astatic primary deflection field separates the drops into printtrajectories and catch trajectories in dependence upon the charge levelwhich they carry and in which a cyclically varying secondary field,extending parallel to the primary deflection field, separates the dropshaving the first non-zero charge level into varying trajectories independence upon the strength of the secondary field at the time that thedrops pass therethrough; to provide such a printer in which a pluralityof jet drop streams are arranged in a row, generally inclined withrespect to the direction of movement of the print receiving medium, andthe deflection of drops by the fields occurs in a directionsubstantially normal to the row; and to provide such a printer in whichthe fields extend perpendicular to the row of jet drop streams.

Other objects and advantages of the invention will be apparent from thefollowing description, the accompanying drawings and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view, illustrating the ink jet printerof the present invention;

FIG. 2 is a sectional view, taken generally along line 2--2 in FIG. 1,with the upper portion of the print head broken away;

FIG. 3 is a complete sectional view of the print head, taken generallyalong the line 2--2 in FIG. 1;

FIG. 4 is a schematic view of a portion of the print receiving medium,as seen from above, illustrating the positions at which drops aredeposited by the jet drop streams;

FIG. 5 is an enlarged sectional view, similar to FIG. 3, of the lowerportion of the print head and a modified secondary deflection electrodearrangement;

FIG. 6 illustrates a cyclically varying secondary deflection potentialvoltage; and

FIG. 7 illustrates fluctuation of the charge electrode voltage on asingle electrode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention relates to ink jet printers and, moreparticularly, to an improved arrangement for selectively depositingdrops from each of a number of jet drop streams at a plurality of printposition on a moving, print receiving medium. As seen in FIGS. 1-3, thefluid jet print head has a manifold means, including upper manifoldportion 10 and lower manifold portion 12, which defines an elongatedcavity 14 therein. Manifold portions 10 and 12 are held together bybolts 16, compressing a sealing ring 18 therebetween.

The print head further includes an orifice plate 20 which defines aplurality of orifices 22 arranged in at least one relatively long row.Orifice plate 20 is mounted on the bottom of manifold portion 12 by anadhesive or, alternatively, by soldering or other appropriate means. Theorifices 22 communicate with cavity 14 and the row of orifices extendsgenerally parallel to the direction of elongation of the cavity 14.

A stimulator 24 is mounted in cavity 14 and, as shown in FIG. 3, isspaced from the orifice plate 20 so as to define a fluid reservoir 26.The stimulator 24 includes a plurality of piezoelectric means defined byelongated transducer 27. The piezoelectric means lengthen and contractvertically when electrically excited with an oscillating signal. Thestimulator further includes acoustic isolation material 28 whichsurrounds the piezoelectric means and provides a means of supporting thepiezoelectric means in the cavity 14.

The oscillatory movement of the bottom surfaces of the piezoelectricmeans produces pressure waves of substantially uniform phase front inthe fluid in the reservoir 26. These waves travel downward through thefluid and are coupled to the field filaments flowing through theorifices 22, causing them to break up into jet drop streams. Anoscillating excitation signal is produced by an electrical signalgenerator 29 and this is applied to the transducer 27 to produce thisoscillatory movement.

Fluid is supplied to the reservoir 26 by a fluid supply inlet 32 whichextends downward through upper manifold portion 10 and a support plate33, attached to manifold portion 10 by bolts 34. Inlet 32 terminates ina channel 36 which extends substantially the entire length of thereservoir 26. A similar channel 38 communicates with the reservoir 26and a fluid outlet 40, providing a means of removing fluid from theprint head during cross flushing at shutdown. The transducer 27 definesa plurality of slots 42, as seen in FIG. 1, which extend alternatelyfrom opposite sides of the transducer partially therethrough so as todefine the plurality of piezoelectric means. The slots 42 reducesubstantially the possibility of wave movement or bending along thelength of the transducer 27.

The electrical signal generator 29 is coupled by means of a conductor 44to a plurality of electrodes 46 on the top surfaces of the transducer27, with the electrodes 46 being connected in parallel by means ofconductors (not shown) which bridge the slots 42. A conductor 50connects generator 29 to conductive fluid in the reservoir 26 viaelectrically conductive manifold portion 12. The fluid contacts thesurfaces 30 (FIG. 5) on the bottom of the transducer and effectivelyacts as a second set of electrodes, opposing electrodes 46. Theacoustical isolation material, of low density, surrounds the transducer27 and effectively isolates it from manifold portion 10. The material 28pots the transducer into position in the cavity 14. A room temperaturevulcanized silicone 53 extends across and into slots 42, as indicated at54, as well as covering isolation material 28. This seals the ink inreservoir 26. A layer of epoxy 55 may be provided in cavity 14 as abacking material for the stimulator. The print head means produces aplurality of jet drop streams when ink is supplied to the reservoir 26under pressure and the stimulator arrangement produces break up of thestreams 57 into drops 58.

A print receiving medium, such as a sheet of paper 60, is transportedpast a print station, indicated generally at 62, by means of atransport, such as belt conveyor 64. The jet drop streams are arrangedin a row 66 which is inclined with respect to the direction of movementof the medium 60.

As is known, the drops 58 may be electrically charged by means ofU-shaped charge electrodes 68 partially surrounding the tips of thefluid filaments which emerge from the orifices 22. The charge electrodes68 provide a means for selectively charging drops in the streams 57 suchthat selected ones of the drops carry a print charge level and theremainder of the drops carry a catch charge level. The charge levelcarried by a drop is controlled by applying a charge potential to theelectrode associated with the fluid filament from which the drop isformed. Induced in the tip of the fluid filament is a charge of oppositepolarity and proportional in magnitude to the electrical potentialplaced on the charge electrode. As the drop is formed from the tip ofthe fluid filament, the drop carries away this induced charge.

The charge electrodes 68 are defined by electrically conductive materiallining notches in charge electrode plate 70. Electrical conductors (notshown), plated on the surface of the electrically non-conductive chargeelectrode plate 70, provide a means of supplying electrical chargepotentials to the various charge electrodes 68 in a controlled,selective manner. The charge potentials are generated by chargingcircuit 72 (FIG. 2) and are supplied to the conductors on plate 70 bymeans of electrically conductive cable 74. The individual chargeelectrodes receive voltage signals which fluctuate selectively between arelatively high catch charge potential and a relatively low print chargepotential, as shown in FIG. 7, based upon whether the drop then beingformed is to be deposited on the print receiving medium. As may be notedin FIG. 7, the charge electrode catch and print potentials are bothnon-zero, such that all drops will carry some electric charge. Asdescribed more fully below in conjuction with FIG. 5, however, the dropswhich are to be caught may, in other embodiment of the invention, carrya zero charge level.

The printer further includes secondary deflection electrode plates 76and 78 which, in combination with secondary deflection potentialgenerator 80, provide a means for generating a cyclically varyingelectrical deflection field which is normal to the row of jet dropstreams and which deflects drops carrying the print charge level intoselected print trajectories. As illustrated in FIG. 6, the voltagedifferential placed across deflection plates 76 and 78 is cyclicallyvaried by generator 80 in a stepwise fashion. As a consequence, thefield extending between plates 76 and 78 is varied in a manner which,although not precisely stepwise due to the capacitive nature of thedeflection electrode structure, nevertheless approximates this stepwisevariation. The drops carrying a print charge level, therefore, aredeflected by varying amounts in dependence upon the field strength atthe time that they pass between the plates. This deflection results inthe drops striking the print receiving medium 60 at corresponding printpositions 82. It will be appreciated, of course, that drops carrying thelarger catch charge level will also be deflected by the field betweenplates 76 and 78. Due to the fact that these drops are subsequentlycaught, this deflection is of no importance. It will be furtherappreciated that the voltage differential between plates 76 and 78 couldbe varied in a manner approximating a ramp function. The drops would besubjected to a linearly varying field such that the average fieldstrength experienced by successive drops would differ and a pattern ofprint positions as shown in FIG. 4 would be produced.

The printer includes a means for generating a static electric deflectionfield for separating the drops into print and catch trajectories,including porous metal electrodes 83 and 84, and voltage source 86.Electrodes 83 and 84 are positioned on opposite sides of row 66 andextend generally parallel thereto. Electrodes 83 and 84 cover vacuumchambers 86 and 88 defined by members 90 and 92, respectively. A partialvacuum is applied to chambers 86 and 88 such that drops which strikeelectrodes 83 and 84 are ingested into the chambers 86 and 88, andsubsequently carried away. Due to the fact that the drops 93 which areto be caught carry a higher charge level than the print drops, drops 93are deflected by the field between plates 83 and 84 to a substantialdegree. This static deflection field is of a higher intensity than thefield between plates 76 and 78 and, further, drops are subjected to thestatic field for a much greater period of time. As a consequence, thedrops 93 are deflected sufficiently so as to be intercepted by the lip94 of catcher 96.

The catcher 96 extends generally parallel to row 66, and is positionedto intercept drops in the catch trajectories and prevent the intercepteddrops from striking the print receiving medium 60. Drops which strikecatcher 96 are carried away by an appropriate liquid suction arrangementand may be collected and returned to the print head 12 for reuse.

The present invention operates by selectively charging drops in the jetdrop streams 57 in a binary fashion, i.e. a relatively low charge levelbeing induced on drops which are to be printed on the medium 60, and arelatively high charge level being induced upon drops 93 which are to becaught by catcher 96. The drops which are to be printed are thendeflected into four different print trajectories by a cyclically varyingsecondary deflection field which is impressed between plates 76 and 78and which produces varying amounts of deflection in accordance with thelevel of the field as successive print drops pass through it. A somewhatgreater deflection of the print drops occurs as they pass betweendeflection electrodes 83 and 84, and are subjected to a higher intensitystatic electric deflection field. This deflection, however, is notsufficient to cause the print drops to be deflected to catcher 96. Thedeflection field is, however, sufficient to deflect the more highlycharged drops 93 to the catcher 96.

As noted previously, the row of jets 66 is skewed with respect to thedirection of paper movement, as illustrated in FIGS. 1 and 4. Note thatthe points 100 on the print receiving medium, as seen from above, denoteprint positions where drops are deposited from a single jet. Similarly,print positions 102 are serviced by a single jet, print positions 104are serviced by a single jet, and print positions 106 are serviced by asingle jet. Deflection of the drops is accomplished by both the primaryand the secondary deflection fields in a direction, indicated by arrow108, which is substantially perpendicular to the row of jets 66. By thistechnique, drops may be deposited along print lines 110 as the printreceiving medium is transported past the print station 62. Collectively,therefore, the deposited drops define a print image on the paper.

Reference is now made to FIG. 5, which illustrates a further embodimentof the invention. The print head and charge electrode plate for thisembodiment are substantially the same as that described with respect tothe embodiment of FIG. 1. It should be noted, however, that a modifiedsecondary deflection electrode structure is provided. This structureincludes electrode plates 112 and 114 which correspond generally toplate 76 and 78, and which are electrically connected to a secondarydeflection potential generator, such as generator 80 in FIG. 2. Theplates 112 and 144 are sandwiched between insulator plates 116 and thisstructure, in turn, is sandwiched between grounded shield plates 118.Plates 118 have the effect of confining the secondary deflection fieldto the immediate region of plates 112 and 114 and thereby prevent thesecondary deflection field from inadvertently affecting charging of thedrops by electrodes 68 or deflection of the drops by the static, primarydeflection field. After passing through the cyclically varying secondarydeflection field, the drops emerge and pass through the primary fieldwhich is defined between grounded plate 120, an extension of a shieldlayer 118, and deflection electrode 122. Electrode 122 receives adeflection potential from voltage source 124.

In this embodiment, drops which are to be caught are not charged and,therefore, pass unaffected through both the primary and secondarydeflection fields, striking catcher 126 which is positioned directly inthe path of the undeflected jet drop streams.

Catcher 126 defines a lip 128 which extends along the row of jet dropstreams, generally parallel to the row. Drops which are to be printed onthe print receiving medium 60 are deflected initially by the fieldbetween plates 112 and 114 into a number of differing printtrajectories. All of these drops are subsequently shifted in trajectoryby the field between electrodes 120 and 122 to a degree that they do notstrike the catcher 126 and are therefore deposited on the printreceiving medium. Note that while the drops which are to be caught inthe embodiment of FIG. 5 carry no electric charge, print drops, likethose produced in the embodiment of FIG. 1, all carry a non-zero chargelevel which permits the drops to be deflected to various print positionson the medium 60 by the cyclically varying secondary deflection field.

It may be desirable with either embodiment to interpose guard dropsbetween successive print drops produced by the print head. A guard dropis simply a drop which is charged to a catch charge level, regardless ofthe image to be printed. Guard drops serve the function of physicallyseparating the drops which may be selectively deposited on the printreceiving medium. By inserting one or more guard drops between eachsuccessive print drop, the print drops are spaced apart sufficiently toinsure that the secondary deflection field acts only on one print dropin each jet drop stream at any given time. Guard drops also function toreduce the electrostatic and aerodynamic effects which drops within astream would otherwise have on each other.

It may be appreciated that although binary charging is utilized in thedescribed embodiments, other charging techniques could be utilized. Forinstance, two charge levels could be selectively induced on drops whichare to be printed on the medium, with a third, higher charge level beinginduced on drops which are to be caught by the catcher. With thistechnique, fewer steps in the secondary deflection voltage are requiredto service the same number of print positions.

While the forms of apparatus herein described constitute preferredembodiments of this invention, it is to be understood that the inventionis not limited to these precise forms of apparatus, and that changes maybe made therein without departing from the scope of the invention whichis defined in the appended claims.

What is claimed is:
 1. An ink jet printer for printing on a movingprint-receiving medium, comprising:print head means for producing aplurality of jet drop streams directed at said medium, said streamsbeing arranged in a row which is oblique with respect to the directionof movement of said medium, charging means for selectively inducing afirst, non-zero charge level on drops which are to be deposited on saidmedium and for inducing a second charge level on drops which are not tobe deposited on said medium, means for providing a static primarydeflection field extending parallel to said medium and perpendicular tosaid row and positioned in the paths of said streams, said primarydeflection field separating said drops into print trajectories and catchtrajectories in dependence upon whether said drops carry said first orsecond charge levels, respectively, catcher means, extending parallel tosaid row of jet drop streams and positioned generally between said printhead means and said medium, for catching drops in said catchtrajectories, and means for providing a cyclically varying secondarydeflection field extending parallel to said primary deflection field andpositioned in the path of said streams, said secondary deflection fieldseparating drops having said first non-zero charge level into varyingtrajectories in dependence upon the strength of the secondary deflectionfield at the time that the drops pass therethrough, whereby the dropsfrom each jet drop stream may be deflected into at least two printtrajectories for deposit at two positions on the print receiving medium.2. An ink jet printer according to claim 1, in which said charging meansincludes means for periodically charging drops to a guard charge level,whereby each jet drop stream includes one or more guard drops betweensuccessive print drops.
 3. An ink jet printer, comprisingmeans fortransporting a print receiving medium past a print station, print headmeans for generating a row of jet drop streams directed at said mediumat said station, said row being inclined with respect to the directionof movement of said medium, charging means for selectively chargingdrops in said streams such that selected ones of said drops carry aprint charge level and others of said drops carry a catch charge level,means for generating a cyclically varying electric deflection field,normal to said row, for deflecting drops carrying said print chargelevel into selected print trajectories, means for generating a staticelectric deflection field normal to said row for separating the dropsinto print and catch trajectories, and a catcher positioned to interceptdrops in said catch trajectories and prevent the intercepted drops fromstriking said print receiving medium.
 4. The printer of claim 3 in whichsaid charging means includes means for inducing a non-zero print chargelevel.
 5. The ink jet printer of claim 4 in which said charging meansfurther includes means for inducing a non-zero catch charge level whichdiffers substantially from said non-zero print charge level.
 6. The inkjet printer of claim 4 in which said charging means further includesmeans for inducing a zero catch charge level.
 7. The ink jet printer ofclaim 4 further comprising means for subjecting only one print drop ineach stream to said cyclically varying electrical deflection field atany given time.
 8. The ink jet printer of claim 7 in which said meansfor generating a cyclically varying electrical deflection field furtherincludes shielding means for limiting said field.
 9. The ink jet printerof claim 4 in which said catcher is positioned to one side of said rowof jet drop streams and extends parallel to said row.
 10. The ink jetprinter of claim 9 in which said cyclically varying field extendsbetween a pair of secondary deflection electrodes positioned on oppositesides of the row of jets between said charging means and said means forgenerating a static electric deflection field.
 11. The ink jet printerof claim 10 in which said means for generating a cyclically varyingfield includes means for applying a cyclically varying electricalpotential to said secondary deflection electrodes.
 12. The ink jetprinter of claim 3 in which said cyclically varying electric deflectionfield varies in a stepwise manner.